fix(cascade): restore the walked parts' attributes; test empty and traced Top seeds

The part-to-master walk saves the restrictions of the parts it walks through and restores them once the master is materialized. It saved each part's restricted-attribute set by reference, and the walk then extends that same set in place, so the restore changed nothing. The extra attributes only made later copies stricter, so the effect was an extra subquery, but the code did not do what its comment says. It now saves a copy of the set.

Two tests pin down seeds this PR changes: a `Top` over a restriction that matches nothing previews no rows and deletes nothing, and `Diagram.trace` starts from the rows a `Top` seed selects.

Author: dimitri-yatsenko

src/datajoint/diagram.py

@@ -1080,8 +1080,12 @@ def _propagate_part_to_master(self, part_node, master_name, mode, restrictions):
         # the master is materialized. The master then cascades back down to
         # all of its parts.
         intermediates = real_path[1:-1]
+        # Copy each node's attribute set too: the walk below extends it in place.
         saved = {
-            node: (copy.copy(restrictions[node]) if node in restrictions else None, self._restriction_attrs.get(node))
+            node: (
+                copy.copy(restrictions[node]) if node in restrictions else None,
+                set(self._restriction_attrs[node]) if node in self._restriction_attrs else None,
+            )
             for node in intermediates
         }
 

tests/integration/test_cascade_overdelete.py

@@ -75,6 +75,60 @@ class Child(dj.Manual):
     assert sorted(Child.to_arrays("item_id")) == [3, 4]
 
 
+def test_top_seed_that_selects_no_rows_deletes_nothing(schema_by_backend):
+    """A ``Top`` over a restriction that matches nothing seeds an empty cascade:
+    the preview counts nothing and the delete removes nothing."""
+
+    @schema_by_backend
+    class Item(dj.Manual):
+        definition = """
+        item_id : int32
+        """
+
+    @schema_by_backend
+    class Child(dj.Manual):
+        definition = """
+        -> Item
+        child_id : int32
+        """
+
+    Item.insert([(i,) for i in range(3)])
+    Child.insert([(i, 0) for i in range(3)])
+
+    seed = Item & "item_id > 100" & dj.Top(2, order_by="item_id")
+    counts = dj.Diagram.cascade(seed).counts()
+    assert counts[Item.full_table_name] == 0
+    assert counts[Child.full_table_name] == 0
+
+    seed.delete(prompt=False)
+    assert len(Item()) == 3
+    assert len(Child()) == 3
+
+
+def test_top_seed_in_trace(schema_by_backend):
+    """``trace`` also starts from the rows a ``Top`` selects."""
+
+    @schema_by_backend
+    class Item(dj.Manual):
+        definition = """
+        item_id : int32
+        """
+
+    @schema_by_backend
+    class Child(dj.Manual):
+        definition = """
+        -> Item
+        child_id : int32
+        """
+
+    Item.insert([(i,) for i in range(5)])
+    Child.insert([(i, 0) for i in range(5)])
+
+    traced = dj.Diagram.trace(Child & dj.Top(2, order_by="item_id"))
+    assert traced.counts()[Child.full_table_name] == 2
+    assert traced.counts()[Item.full_table_name] == 2
+
+
 def test_nullable_fk_sharing_a_primary_key_column(schema_by_backend):
     """A child whose nullable foreign key shares a primary-key column with the
     parent keeps the rows whose foreign key is NULL: they reference nothing."""